This invention relates to a multilayer ceramic circuit board for use in mounting a semiconductor chip or chips thereon and, in particular, to a multilayer ceramic circuit board fired at a low temperature which may be between 800.degree. C. and 1000.degree. C.
As a rule, a multilayer ceramic circuit board of the type described comprises a plurality of ceramic layers which are stacked together and one of which serves as a substrate. A conductive pattern is deposited or printed on the substrate and is often electrically connected through via holes to another conductive pattern formed on the other ceramic layer that will be called an insulating layer for insulating the conductive patterns from each other. A resistor pattern is often printed on a selected one or ones of the surfaces and is covered with an insulating protection layer to be buried in the multilayer circuit board. The insulating protection layer may be called an insulating protection layer.
In a conventional multilayer ceramic circuit board, each of the ceramic layers and the protection layer is formed by a ceramic material which can be fired at a high temperature, such as 1600.degree. C. In this connection, the conventional multilayer ceramic circuit board may be called a high temperature fired board. The ceramic material has a high dielectric constant. The high temperature fired board needs to use the conductive patterns of a high refractory metal, such as molybdenum or tungsten, having a comparatively high resistivity. Such a high dielectric constant of the ceramic material and such a high resistivity of the high refractory metal lengthen a signal transmission time of an electric signal transmitted through the conductive pattern and become a bar to a high speed operation and a high performance of a computer which uses a great number of multilayer ceramic circuit boards of the type described.
In order to improve the above-mentioned shortcomings of the high temperature fired circuit board, recent interest is directed to a low temperature fired circuit board which is fired at a low temperature less than 1600.degree. C. and, preferably, between 800.degree. C. and 1000.degree. C.
As mentioned before, a resistor pattern is often buried in a multilayer ceramic circuit board and is covered with an insulating protection layer. The resistor pattern is composed of a glass composition and RuO.sub.2 or Bi.sub.2 Ru.sub.2 O.sub.7 while the insulating protection layer is usually composed of the same material as the underlying ceramic layer. It is to be noted that a resistance value of the resistor pattern is liable to vary from a desired value during a manufacturing process and that the resistor pattern must be trimmed by the use of a laser beam to adjust the resistance value to the desired one.
When the insulating protection layer is formed by a low temperature fired ceramic material, as described before, it has been found out that the insulating protection layer is inconvenient for trimming the conductive pattern by the laser beam because the laser beam is not easily absorbed into such an insulating protection layer. As a result, the trimming process for the resistor pattern is time-consuming.
In U.S. patent application Ser. No. 740,184, a low temperature fired circuit board is proposed by S. Nishigaki et al and comprises a ceramic layer or layers produced from a mixture of alumina particles and a glass composition comprising by weight 45% to 70% of SiO.sub.2, 0% to 30% of Al.sub.2 O.sub.3, 0% to 30% of B.sub.2 O.sub.3, and 10% to 55% of CaO and MgO. More particularly, each ceramic layer comprises coexistence of a glass portion uncrystallized and alumina remaining unmelted in the glass portion and is thus formed by a low temperature fired ceramic material. The ceramic layer enables a resistor pattern to be easily trimmed by the laser beam when used as the insulating protection layer.
Such a low temperature fired circuit board can use the conductive patterns of a low resistivity metal, such as silver, gold, platinum. Inasmuch as the low temperature fired ceramic material has a low dielectric constant, it is possible to shorten the signal transmission time by the use of the low temperature fired circuit board.
However, it has been found out that the low temperature fired circuit board is weak in adhesion between the conductive patterns and the ceramic layer. Such adhesion is seriously degraded often a thermal aging process of about 150.degree. C. or so. Besides, such a circuit board is undesiredly warped because shrinkage of the conductive patterns is largely different from shrinkage of the ceramic layers when they are cofired together.
In order to improve adhesion between a ceramic material and a conductive material, an additive, such as glass frit, is often added to the ceramic material, as known in the art. Such addition of glass frit makes wettability of the conductive material to solder become worse and makes warp of a ceramic layer large, when the glass frit is used as the additive on manufacturing the low temperature fired circuit board. Alternatively, it is also known in the art to add, to the ceramic material, another additive, such as CuO, reacting with an alumina component of the ceramic material. However, addition of CuO is scarcely effective to increase the adhesion between the ceramic and the conductive materials because CuO is almost dispersed into the glass portion.